The devices utilizing cellular phone technology, such as cellular phones and like, do more commonly possess additional integrated features for short range wireless communication with other devices. Generally these kinds of wireless communication technologies are meant for short distance low-power communication, such as communication in a single room or in a building. Two common such wireless technologies are Wireless Local Area Network (WLAN) and Bluetooth. As WLAN is generally a wireless Ethernet, Bluetooth in an open specification designed to replace cables between cell phones, laptops and other computing and communication devices within a 10 meter range. Generally they both operate at the same free 2.4 GHz or 5.7 GHz band.
Wireless Local Area Networks
Wireless local area networks become more common, and ever more terminals of different types can communicate in them. Traditionally wireless local area networks have been used mainly by portable computers or the like, but in an ever increasing manner wireless local area network properties are also included in other mobile terminals, for instance in different palm computers or mobile phones, such as in a GSM telephone or a communicator.
The present Wireless Local Area Networks (WLAN) are mainly based on the standard IEEE802.11. WLAN provides a network connection, which works over a relatively large area without inconvenient cables. Typically a WLAN operates for instance in an office environment or within a building, whereby it provides a possibility for e.g. portable computers or the like to make a wireless connection to the local area network of a company or other service provider. The device making a connection to the network can be for instance a separate connectable WLAN card, or the WLAN properties can be already integrated in the device.
A WLAN can either be formed of terminals, which use only wireless network connections, or it can exist as an extension of a wired network. A WLAN comprising only wireless terminals is usually called an ad hoc wireless local area network, as there are no other available communication methods in addition to the WLAN equipment to form the local area network. A network is called an infrastructure WLAN if other communication means are available in addition to the wireless communication techniques.
Thus the independent or ad hoc WLANs communicate only with other WLAN devices within their own wireless network environment. An ad hoc WLAN functions so that each network terminal can transmit data directly to another WLAN device within the same ad hoc network (point-to-point communication), and it does not require the data to pass other equipment. This is a useful solution when the network is formed within a small group, for instance for different meetings, whereby only a few work stations or other terminals are used. It is easy to establish an ad hoc network, and it does not require complicated network management, but it is not suitable for larger entities. An ad hoc network can be expanded by using an additional base station (AP, Access Point), whereby the base station functions as a repeater, which receives information and transmits this information to all WLAN terminals within its coverage area. This makes it possible to cover a larger area, even of the double size within the network.
Infrastructure WLAN refers to a wireless local area network, which is connected with at least one base station to a wired local area network. The area of the base station can include several terminals (Stations, STA), which can communicate with the whole network via the base station. If there is only one base station, then the network is called a Basic Service Set (BSS) WLAN. If the network contains several BSS sets, which together form a sub-network, then this is called an Extended Service Set (ESS) WLAN. A bus between base stations in the ESS networks is called a Distribution System (DS), which can be for instance an Ethernet system or a wireless system. The area covered by one base station is called a microcell. A WLAN comprising a distribution system, a base station or base stations with their microcells presents itself as one network of the IEEE802 standard to the higher layers of the OSI layer model. If a BBS forms an independent network without connections to a distribution system it is called Independent Basic Service Set (IBSS) (in an ad hoc network).
A WLAN according to the standard IEEE802.11 can be built using radio frequency techniques, such as narrow band radio frequency techniques and spread spectrum techniques, such as Frequency-Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum (DSSS).
All stations (STA) within the coverage area of one BBS must be synchronized to the same clock, which is usually made with the aid of a Timing Synchronization Function (TSF). In infrastructure WLANs a base station (AP) maintains this timing and performs the TSF. The base station transmits periodically certain frames, which are called beacons, and which contain data about the TSF clock, so that other stations within the same BBS can be synchronized. A receiving station must always accept a beacon sent by the base station of the BBS which serves this station. If the TSF clock in the station differs from the received data in the beacon, then the receiving station sets its clock to the received value. The base station generates and transmits a beacon once in a time unit, which is called the Beacon Period.
In an IBSS network the TSF is realized by a distributed algorithm, which is realized by all members of the BBS. According to this algorithm each station of the BBS transmits beacons, and each station in the IBSS adjusts its clock on the basis of the timing of that beacon with a TSF value, which is later than the station's own value.
The base station shall define the timing for the entire BBS by transmitting beacons according to the aBeaconPeriod attribute within the base station. This defines a series of Target Beacon Transmission Times (TBTT), which exactly aBeaconPeriod time units apart. Time zero is defined to be a TBTT with the beacon being Delivery Traffic Indication Message (DTIM) and which is transmitted at the beginning of each Contention-free Period (CFP). At each TBTT the base station shall schedule the beacon as the next frame for transmission.
In a wireless local area network a station (terminal) can switch into a Power Save (PS) mode. For instance in infrastructure WLAN a station then informs the base station about this by using the power management bits in the Frame Control field. Then the base station will not arbitrarily transmit data (MSDU, MAC Service Data Unit) to such stations, but it buffers the data and transmits it only at a certain moment. Such stations, to which data has been buffered in the base station, are marked in the Traffic Indication Map (TIM), which is included as one element in each beacon created by the base station. By receiving and interpreting the TIM a station will detect that there exists buffered data intended for it. Stations operating in the power management mode will periodically listen to the beacons, according to what is defined by the parameters ListenInterval and ReceiveDTIM of the station's power management.
When a station in power management mode detects that data for it is buffered in the base station it will transmit a short PS-Poll frame to the base station, which in turn will immediately respond by the data in question, or it states that it has received the PS-Poll and will transmit the data later on. If a TIM informing of buffered data is transmitted during a CFP, then a station in the power save mode will not transmit a PS-Poll frame, but remains active until the buffered data has been received (or the CFP is finished). If a station of the BSS is in the power save mode the base station will buffer all broadcast and multicast MSDUs and send them to all stations immediately after the next beacon, which contains a DTIM transmission.
A station (STA) remains in the current power management mode until it informs the base station about a changed power management mode initiated by the station itself by successfully exchanging frames. The power management mode must not change during a period of frame exchange.
The station can have two different power states: awake, whereby it is fully powered, or doze, whereby the station is not able to transmit or receive, and whereby it consumes very little power. The manner in which the station switches between these two states depends on the station's power save mode; the modes are the Active Mode (AM) and the Power Save (PS) mode.
In the active mode the station is able to receive frames at any time, and it is in the state “awake”.
In the power save mode the station listens to certain beacons (depending on the ListenInterval parameter of the MLME-Associate.request element) and transmits PS-Poll frames to the base station, if the TIM element in the most recent beacon shows that data (MSDU) for the station has been buffered. The base station transmits buffered MSDUs intended for a station in power save mode only as a response to a PS-Poll from this station, or during CFP if the station is a CF-Pollable PS-station. In the power save mode the station is in the doze state, and switches into the active state in order to receive beacons, to receive broadcast and multicast transmissions, which follow certain received beacons, to transmit, and to wait for responses to the transmitted PS-Poll frames or (concerning CF-Pollable stations) to receive contention-free transmissions or buffered MSDUs.
When a station is in the course of switching from the doze state to the ‘awake’ state in order to transmit it will first assess the state of the channel by a Clear Channel Assessment (CCA) procedure, until it detects a frame sequence where it can correctly set its Network Allocation Vector (NAV), or until a period corresponding to ProbeDelay expires.
TIM identifies those stations, which are about to receive traffic, which has been buffered in the base station. Further the TIM contains information about any expected broadcast/multicast traffic. The base station allocates an Association ID (AID) to each station. AID 0 (zero) is reserved to identify a buffered broadcast/multicast MSDU.
There are two types of TIM: TIM and DTIM. After a DTIM the base station transmits the buffered broadcast/multicast MSDUs using normal frame transmission rules, and before it transmits any unicast frames. The base station transmits a TIM in each beacon. During each DTIM period (DTIMPeriod) a TIM of the ‘DTIM’type is transmitted in the beacon instead of a normal TIM.
The stations in power save mode act in the following ways in order to receive, depending on whether it is a Contention Period (CP) or Contention-Free Period (CFP).
FIG. 1 shows the functions of a base station and stations in a case when a DTIM is transmitted always after two TIMs. The top segment of a line is the time axis, which shows the intervals between the beacons and between the DTIMs. The second segment of a line represents the base station's functions. The base station synchronises the beacons to be transmitted at each beacon interval, but the transmission of the beacons may be delayed, if there is traffic during the TBTT. This is marked in the figure with the text ‘Busy Medium’. The third and fourth segments of a line represent stations, which are in different power save modes. Both stations switch on their receivers when they must listen to the TIMs. This is drawn in the figure with the aid of rising symbols representing the power of the receiver. For instance, the station shown on the third segment of a line switches on its receiver in order to receive the TIM transmitted already during the first beacon. The TIM shows that there is a buffered MSDU for the receiving station. The receiving station generates a PS-Poll frame, which causes the transmission of the buffered data from the base station. Broadcast and multicast MSDUs are transmitted from the base station after a beacon containing a DTIM. The station shown on the fourth segment of a line operates in a state consuming extremely little power, and in the figure it switches on its receiver only once in order to receive a TIM.
When the Point Coordinator (PC) is inactive, and during CP when the PC is active, the station (STA) awakes from its doze state sufficiently early in order to receive the next agreed beacon. When the station detects that the AID bit corresponding to this station is set in the TIM, then the station starts a PS-Poll in order to obtain the buffered MSDU of a management frame. If more bits are set in the TIM the PS-Poll is transmitted after a random period. The station remains in the ‘awake’ state until it receives a response to its poll, or until it receives another beacon with a TIM indicating that the base station has buffered MSDUs or management frames for this station. If the AID bit corresponding to this station is set in the next TIM the station starts a new PS-Poll in order to obtain the buffered MSDU(s) or management frame(s). If the More Data field in the received MSDU or management frame shows that more traffic has been buffered for this station, then the station transmits a new poll at a suitable moment, until no more MSDUs or management frames are buffered for this station. If the ReceiveDTIM is true, then the station awakes from its doze state sufficiently early in order to receive each DTIM. A station receiving broadcast/multicast MSDUs remains in the ‘awake’ state until the More Data field or the broadcast/multicast MSDUs show that there are no more buffered broadcast/multicast MSDUs, or until it has received a TIM, which shows that there are no more buffered broadcast/multicast MSDUs.
In a BBS with an active PC the station awakes during the CFP to receive a beacon (containing the DTIM) at the beginning of each CFP. In order to receive broadcast/multicast MSDUs the station awakes sufficiently early in order to receive each DTIM, which may be transmitted during the CFP. A station receiving broadcast/multicast MSDUs remains in the ‘awake’ state until the More Data field of the broadcast/multicast MSDUs shows that there are no more buffered broadcast/multicast MSDUs, or until it has received a TIM, which shows that there a no more buffered broadcast/multicast MSDUs. When a station detects that the AID bit corresponding to this station is set in the DTIM at the beginning of the CFP (or in the next TIM during CFP) it will remain in the ‘awake’ state during at least a portion of the CFP until the station receives a MSDU or management frame directed to it from the base station and the More Data field in the Frame Control field shows that no further traffic has been buffered. If the More Data field in the Frame Control field in the last MSDU or management frame received from the base station shows that there exists more buffered traffic for the station, then the station can remain in the ‘awake’ state when CFP is terminated, and then it can send PS-Poll frames during the CP in order to request these extra MSDUs or management frames, or the station can switch itself into the doze state during CP (except during TBTT when DTIMs are expected during CP) in order to wait for the start of the next CFP.
A station in the active state keeps its receiver continuously active, and it does not have to assess the traffic information part of the beacon.
The base station has an expiration function which removes buffered traffic, which has been buffered over an unreasonably long time. This function is based on the ListenInterval parameter of the MLME-Associate.request primitive of that station, for which the traffic is buffered.
The description above refers mainly to infrastructure WLAN systems. The power management in IBSS systems differs slightly from it, even if the basic idea is similar. The stations are synchronized, and information about multicast MSDUs and MSDUs to be transmitted to stations in the power save mode is first presented during that period when the stations are in the ‘awake’ state. The announcement is made with the aid of ad hoc Traffic Indication Messages (ATIM). A station in the power save mode listens to these messages so that it can determine whether it should stay in the ‘awake’ state.
When an MSDU is transmitted so that it is addressed to a certain station being in the power save mode, then the transmitting station first transmits an ATIM frame within the ATIM window, during which all stations are in the ‘awake’ state, also those in the power save mode. The ATIM window is defined as a specific period of time (defined by aATIMWindow), following a TBTT, and during which only beacons or ATIM frames shall be transmitted. The addressed ATIMs shall be acknowledged. If the station transmitting a directed ATIM does not receive an acknowledgement it shall execute the backoff procedure for retransmission of the ATIM. Multicast ATIMs shall not be acknowledged.
After the ATIM interval only those directed MSDUs, that have been successfully announced with an acknowledged ATIM, as well as the broadcast/multicast MSDUs announced by ATIMs shall be transmitted to the stations, which are in the power save mode. The transmission of these frames shall be done using the normal DCF access procedure.
Bluetooth
The Bluetooth (Trademark) standard (see Bluetooth specification) describes how low power radio transceivers can be used to remotely communicate over a range of tens of meters. These low power transceiver devices are already present in some mobile phones and can be used to allow a user input, such as a cordless headset, to be used remotely from the mobile phone. Communication between the headset and the phone occurs between a low power radio transceiver in the headset and the low power radio transceiver in the phone. A particular advantage of Bluetooth transceivers in mobile applications is that they have energy conservation modes which prolong battery life.
Two or more units sharing the same Bluetooth channel form a piconet. One Bluetooth unit acts as the master of the piconet, whereas the other unit(s) acts as slave(s). Up to seven slaves can be active in the piconet. Multiple piconets with overlapping coverage areas form a scatternet. Each piconet can only have a single master. However, slaves can participate in different piconets on a time-division multiplex basis. In addition, a master in one piconet can be a slave in another piconet. The piconets shall not be time- or frequency-synchronized. The piconet is synchronized by the system clock of the master. The master never adjusts its system clock during the existence of the piconet. The slaves adapt their native clocks with a timing offset in order to match the master clock.
Features are included into Bluetooth to ensure a low-power operation. These features are both at the microscopic level when handling the packets, and at the macroscopic level using certain operation modes. Three modes are described during the connection state (wherein the connection has been established and packets can be sent back and forth) which reduce power consumption: sniff mode with higher duty cycle, hold mode with lower duty cycle and park mode with the lowest duty cycle.
In active mode, the Bluetooth unit actively participates on the channel. The master schedules the transmission based on traffic demands to and from the different slaves. In addition, it supports regular transmissions to keep slaves synchronized to the channel. Active slaves listen in the master-to-slaves slots for packets.
In the sniff mode, the duty cycle of the slave's listen activity can be reduced. If a slave participates on an ACL link, it has to listen in every ACL slot to the master traffic. With the sniff mode, the time slots where the master can start transmission to a specific slave is reduced, i.e. the master can only start transmission in specified time slots. These so-called sniff slots are spaced regularly with a interval of Tsniff (herein called ‘sniff intervals’). To enter the sniff mode, the master shall issue a sniff command via the LM protocol.
During the connection state, the ACL link to a slave can be put in a hold mode. This means that the slave temporarily does not support ACL packets on the channel any more. With the hold mode, capacity can be made free to do other things like scanning, paging, inquiring or attending to another piconet. During the hold mode the slave unit keeps its active member address (AM_ADDR). Prior to entering the hold mode, master and slave agree on the time duration the slave remains in the hold mode. When the timer is expired, the slave will wake up, synchronize to the traffic on the channel and will wait for further master instructions.
When a slave does not need to participate on the piconet channel, but still wants to remain synchronized to the channel, it can enter the park mode which is a low-power mode with very little activity in the slave. In the park mode, the slave gives up its active member address (AM_ADDR). The parked slave wakes up at regular intervals to listen to the channel in order to re-synchronize and to check for broadcast messages. To support the synchronization and channel access of the parked slaves, the master establishes a beacon channel when one or more slaves are parked. The beacon channel serves four purposes: transmission of master-to-slave packets which the parked slaves can use for re-synchronization, carrying messages to the parked slaves to change the beacon parameters, carrying general broadcast messages to the parked slaves, and unparking of one or more parked slaves.